Two former JPMorgan Chase & Co. employees were charged by U.S. prosecutors with attempting to conceal trading losses at the largest U.S. bank last year as part of a probe of its $6.2 billion loss on derivatives bets.

Bruno Iksil, the JPMorgan Chase & Co. trader known as the London Whale, had an e-mail sent to about 20 colleagues last year estimating almost double the daily losses his book had shown thus far. Then the phone rang.